Analyte detection technology is currently employed in a wide range of disciplines ranging from electrochemical analysis through measurements to detect the presence and amount of biological compounds to pollution monitoring and industrial control. For example, chemical sensors have been developed to determine carbon dioxide levels in underground parking structures and in industrial manufacturing plants and to detect the presence of certain toxic chemicals in homes and in mines. Federal, state, and local governments have become increasingly aware of the dangers posed by many airborne pollutants and have begun to regularly monitor the levels of pollutants using chemical sensors. In addition, the threat of terrorist attacks employing toxic chemical weapons has created public concern and a demand for chemical sensors that can detect particular chemical weapons so that government authorities can respond accordingly. In the medical fields sensors have also been developed to detect quantities of certain biological compounds.
Although advancements in engineering and scientific disciplines have made it possible to fabricate chemical sensors to detect a variety of different analytes, a typical chemical sensor is often limited to detection of a single analyte or a small number of different kinds of analytes. In addition, a number of steps may be needed to prepare an analyte for detection. For example, certain chemical sensors employ a fluorescent material immobilized on an optical-fiber core. An analyte is detected by observing a color change that results from the fluorescent material reacting with the analyte. However, in order to detect a different analyte, the fluorescent material needs to be changed to one that fluoresces when reacted with the different analyte. Certain types of biosensors may employ active biological or biologically derived components which form chemical bonds with an analyte and hold the analyte in position for detection by a chemical sensor. An indirect approach is to use an enzyme that catalyzes a chemical reaction when an analyte is present to produce a product that can be detected by a chemical sensor. The presence of the product is assumed to confirm the existence of the analyte.
In recent years, Raman spectroscopic methods have also been developed to detect analytes. A typically analyte molecule has a unique Raman spectra that can be used to identify the analyte. For example, Raman spectra obtained from gas and liquid phase Raman scattering have been used to identify certain unknown analyte molecules. However, the intensities associated with these Raman spectra are often weak. In more recent years, surface-enhanced Raman spectroscopy (“SERS”) has been developed as an analyte-detection tool. Raman scattering from an analyte adsorbed on or even within a few Angstroms of a metal surface can be 103-106 times greater than Raman scattering observed for the same analyte in gas or liquid phases. SERS enhances the Raman spectra of an analyte via two mechanisms. The first mechanism is an enhanced electromagnetic field called a surface plasmon produced at the surface of a metal. The surface plasmon can be created when the wavelength associated with incident electromagnetic radiation is close to the plasma wavelength of the metal surface. Molecules adsorbed or in close proximity to the surface experience a larger electromagnetic field than the field used to produce the Raman scattering in the liquid and gas phase. The second mechanism of enhancement results from the formation of a charge-transfer complex between the metal surface and the analyte. However, even SERS is not able to provide the enhancement often needed to identify a wide variety of analytes.